1. Technical Field
The present invention relates to a resonant circuit, an oscillation circuit, a filter circuit, and an electronic device. Particularly, the invention relates to a structure of the resonant circuit using a MEMS resonator.
2. Related Art
In the recent electronic device market, products using MEMS (micro electro-mechanical system) technology are becoming increasingly popular. The MEMS products are electro-mechanical devices (MEMS devices) having a micro structure (a MEMS structure) formed on a substrate by using a semiconductor-manufacturing technology. Specific examples of the devices using the MEMS technology include micro sensors such as acceleration sensors, angular velocity sensors, inertial sensors, and pressure sensors. Market demands for the sensors are rapidly growing these days. Thus, the MEMS technology has great potential in creating new devices. In the future, a variety of practical applications of the MEMS devices, alone or in combinations, are expected to be sequentially achieved by utilizing characteristics of micro structures such as micro mechanical relays (switches) and variable capacitance elements.
A new practical example of the MEMS technology is a resonator, which is also called vibrator, oscillator, resonator element, resonator device, or the like, and referred to as “MEMS resonator” in the present specification. The MEMS resonator is typically operated by an electrostatic driving and sensing system or a piezoelectric driving and sensing system. Particularly, the former system is highly consistent with a semiconductor process such as a CMOS process and thus can be regarded as an effective system for miniaturization and cost reduction. In the electrostatic driving and sensing system, there is formed a MEMS resonator with a movable electrode and a fixed electrode to sense electrostatic force-induced vibration of the movable electrode by a change in capacitance between the movable electrode and the fixed electrode. FIGS. 3A and 3B show common examples of transmission characteristics and phase characteristics of such a MEMS resonator. Examples of the resonant circuit shown in the drawings are disclosed in U.S. Pat. No. 6,249,073, U.S. Pat. No. 6,424,074, and JP-A-2004-58228.
In the MEMS resonator as above, usually, a direct current bias voltage is applied between the movable electrode and the fixed electrode of the resonator. Application of the direct current bias voltage facilitates vibration of the resonator, improving resonant characteristics. In this case, the bias voltage needs to be set with a predetermined margin to prevent occurrence of a pull-in phenomenon in which the movable electrode contacts with the fixed electrode due to electrostatic attraction generated between the electrodes of the resonator. Under the circumstances, in order to improve the resonant characteristics of the MEMS resonator, usually, it is desirable to increase the bias voltage while maintaining the margin. However, resonance frequency of the resonator is changed depending on a value of the bias voltage. In other words, a change in the applied bias voltage due to an external factor causes a shift in the resonance frequency of the MEMS resonator. Accordingly, oscillation circuits or filter circuits including a resonant circuit with the MEMS resonator cannot produce a desired oscillation frequency or desired filter characteristics because of the bias voltage change.
Specifically, since the resonance frequency of the MEMS resonator depends on the bias voltage, the bias voltage needs to be set in consideration of the resonance frequency in order to control a changing mode of the resonance frequency. This makes it difficult to use the MEMS resonator in various circuit structures.